In the fabrication of laminated safety glass, it is customary to place a piece of thermoplastic sheeting between two pieces of float glass. It is also common that the thermoplastic interlayer's surface can be roughened to avoid blocking, i.e., one layer of interlayer sticking to another. The roughness on the interlayer can also allow the interlayer to be moved while the two pieces of glass are aligned as the glass/interlayer/glass sandwich (hereinafter, “assembly”) is constructed. In constructing such an assembly, air is trapped in the interstitial space between the glass surface and the bulk of the thermoplastic interlayer. Trapped air can be removed either by vacuum de-airing or by nipping the assembly between a pair of rollers.
The degree to which air must be removed (reduced) from between the glass and interlayer will depend on the nature of the interlayer to absorb the air (dissolution) during the further laminating steps (often an autoclave is used) such that the air forms a ‘solution’ with the interlayer. The presence of a gaseous phase within the laminate will take the form of bubbles or pockets of gas between the interlayer and glass interface. These are generally objectionable for end use applications where the laminate functions as a transparent article, that is, being essentially free of optical defects (e.g. relatively low-haze thus providing a transparent article without hindering visibility). Autoclaving is a step typically utilized in the production of laminated glass using a combination of heat and pressure to hasten the dissolution of any residual air (gaseous component) within the laminate assembly. As external pressure on the laminate is increased (by thermodynamic principals), it restricts the ability for gaseous components to either remain or to form. After laminate processing, the desire is for creation of a ‘solid-phase’ of interlayer, essentially free of a gas phase, is paramount. Additionally, the laminate should remain ‘bubble-free’ for a substantial period of time (years) under end use conditions to fulfill its commercial role. It is not an uncommon defect in laminated glass for dissolved gasses to come out of solution (form bubbles or delaminated areas between the glass/interlayer interface) as time progresses, especially at elevated temperatures experienced in automobiles, buildings and the like, often due to weather conditions and sunlight exposure.
In the case of vacuum de-airing, air is removed while the assembly is at ambient temperature. Tacking of the interlayer to the glass and sealing of the edges is accomplished by heating the entire assembly while it is still under vacuum. The assembly, after the heating step, is generally referred to as a pre-press or a pre-laminate.
In the case of nipping, the assembly is generally heated to a temperature between 50-100° C., and is then passed through one or more sets of nip rolls. Edge sealing is accomplished by the force of the rollers exerted on the two pieces of glass. At the end of the nipping step, the assembly is called a pre-press. In windshield manufacture, the nip rolls are often articulated so as to accommodate the curvature in the windshield. When complex shapes and angles are involved, or when several models of windshields are made concurrently, it is often more convenient to use the vacuum de-airing method.
However, laminators may encounter an issue when selecting a suitable interlayer. It is sometimes difficult to choose an interlayer with optimal features for pre-pressing, namely, rapid air removal and proper edge seal. Interlayers which have rougher surfaces as measured by the 10-point roughness (ISO R468), Rz, can allow for faster de-airing. However, such interlayers can make it inconvenient to obtain edge seal as more energy is generally required to compact the rough interlayer. If the edges of the pre-press are not completely sealed, air can penetrate the edge in the autoclaving step where the pre-press is heated under high pressure, and can cause visual defects in the laminate which is commercially unacceptable. Laminators who use vacuum for de-airing in hot environments can have added difficulty. Interlayers that are rough and allow for rapid de-airing at about room temperature (23° C.) often do not de-air as well when the ambient temperature is much above 30° C.
On the other hand, relatively smooth interlayers can lead to the edges sealing before sufficient air is removed, and can leave air trapped inside the pre-press. This problem is commonly referred to as pre-mature edge seal, and can be especially common with PVB interlayers. During autoclaving, the excess air may be forced into solution under high pressure, but may return to the gas phase after autoclaving. Defects which occur after lamination are often more costly to rectify.
Safety glasses can be obtained using various types of interlayer materials, including, for example: polyvinyl butyral (PVB); polyurethane (PU); ethylene copolymers such as ethylene vinyl acetate (EVA) and/or ethylene/acid copolymers (acid copolymers) and/or ionomeric derivatives thereof (ionomers); and polyvinyl chloride (PVC). Polymeric interlayer materials are thermoplastic. Thermoplastic interlayers are typically heated during the lamination process to soften the interlayer and facilitate adhesion to glass or plastic material. Surface patterns on the interlayers can be provided to allow for rapid de-airing even at high temperatures, and also allow good edge seal to be obtained. Choice or design of an ideal surface pattern can depend on the lamination process parameters as well as on the interlayer material. For example, PVB that is plasticized for use in safety glass is a tacky material that readily adheres to glass even at room temperature. Various surface patterns can be used with PVB, but typically the patterns are designed to account for the physical characteristics of the specific interlayer and/or the specific process.
Ionomer or ionoplastic material (the terms are used interchangeably in the present application, and are considered identical for the purposes of the present invention) is typically not plasticized, and the physical properties of interlayer sheeting obtained from ionomer can be substantially different from the physical properties of other interlayer materials such as PVB. Due to these physical differences, surface patterns useful for plasticized PVB interlayer sheeting may not be ideal for ionomer interlayer sheeting, and vice versa.
The surface patterns for plasticized PVB, for example, tend to be deep to allow air to escape during the lamination process. The broad melting or softening range of plasticized PVB allows the use of such deep patterns. However, the use of deep patterns with ionoplastic interlayers is not trouble-free. The deeper patterns tend to allow more dust or dirt to settle on the surface of the interlayer and can give rise to “pattern haze” in the laminate. Also, the sharper melting range of an unplasticized interlayer can lead to trapped air in the laminate.